This invention relates to a hydraulic shock absorber, and more particularly to a telescopic type hydraulic shock absorber used with various vehicles such as motorcycles, automobiles, and snowmobiles.
It is known that a hydraulic shock absorber included in the suspension system of a vehicle carries out damping by the flow resistance which the damping oil held in a cylinder undergoes when conducted through the restricted passage or passages formed in a reciprocating piston.
With this type of hydraulic shock absorber, absorbed damping energy is converted into heat to raise the temperature of oil to, for example, 150.degree. C at a maximum. The flow resistance of oil is governed by its viscosity which generally decreases with the rise of its temperature.
The restricted passage formed in the piston used with the prior art hydraulic shock absorber is provided with an opening-fixed oil orifice. Therefore, the conventional hydraulic shock absorber set at a prescribed level of temperature (usually, room temperature) declines in the damping property, when the temperature of oil is raised by the long operation of the shock absorber with the resultant fall in the viscosity of oil. In extreme cases, the so-called full bumping arises due to the piston forcefully impinging on the end wall of the cylinder. These drawbacks take place not only by absorption of damping energy, but also by changes in external factors such as seasonal or daily climatic conditions.
To resolve such difficulties, another type of hydraulic shock absorber has hitherto been developed in which a piston passage-restricting member provided with an opening-fixed orifice is detachably fitted to a reciprocating piston. With this type of hydraulic shock absorber, piston passage-restricting members provided with oil orifices having different opening areas are exchanged for each other according to average seasonal atmospheric temperature. However, not only replacement of the piston passage-restricting members calls for very difficult time-consuming work, but also the hydraulic shock absorber provided with said opening-fixed oil orifice fails to cope with the increased temperature of damping oil caused by absorption of damping energy or temporary changes in atmospheric temperature.
It is accordingly an object of this invention to provide a novel hydraulic shock absorber of simple construction which can automatically retain a constant damping property even when the temperature of damping oil changes.
Another object of the invention is to provide a hydraulic shock absorber, whose damping property is kept constant particularly during the initial period in which a shock is applied.
Another object of the invention is to provide a hydraulic shock absorber whose damping property is kept constant particularly during a period following said initial period of shock.